Rotavirus is a large (dia=1000 ) icosahedral virus composed of three capsid protein layers and 11 dsRNA segments. It is the most important cause of gastroenteritis in children accounting for over a million deaths annually. The three-dimensional structure of actively-transcribing rotavirus particles reveals that the nascent mRNA transcripts generated within the core of the virion by endogenous transcriptase complexes are translocated through the intact capsid via a system of channels at the icosahedral 5-fold axes.